Public, private, or government-owned power utility companies generate and distribute power over a distribution infrastructure colloquially called a “power grid”, a “national grid”, or just “the grid”. Power grids typically use high voltage power lines to distribute power over long distances because using higher voltages reduces the energy lost in long-distance transmission. In the United States, for example, long-distance power transmission lines use higher voltages such as 138,000 V and 765,500 V. At the destination, the voltage is stepped down to tens of thousands of Volts for distribution to customer sites, where the voltage is again stepped down to household voltages such as 120V and 240V in the United States and 100V and 200V in other countries.
Traditionally, power grids transmit power as high voltage alternating current, or HVAC. Historically, AC has been used instead of DC for two reasons. First, when power grids were first being built, it was relatively easy to use transformers to step up the voltage of AC lines to create the high voltages necessary to travel enough distance to make distribution feasible, but transformers don't work for DC and there was nothing else at the time that could step up DC voltages to the needed levels. Second, then and now, the vast majority of power is generated by coal-, gas-, or nuclear-filed steam turbines, which are designed to run at a constant speed and which produce AC power at the grid frequency, which is 60 MHz in the United States and 50 MHz in other countries.
Lately, however, interest in alternative energy sources has increased. Wind power, for example, is attractive as a renewable, non-polluting energy source. Unlike steam turbine generators, however, wind-driven turbines, or wind-turbines, do not always run at the same speed; their speed may fluctuate as the wind speed of prevailing winds fluctuates, which makes it difficult to control the frequency of the AC output. This makes wind-driven turbines unsuitable as a power generator in an AC power grid with a fixed frequency and phase.
One solution to the problem of fluctuating wind-turbine speed and AC output frequency is to convert the AC output of the wind turbine to DC via a rectifier and regulate the DC output using a regulator, but the DC output must then be converted back to AC using a device called an inverter and have its voltage stepped up with a transformer before being added to the power grid. Even if the power grid itself is HVDC, the voltage output by the wind turbine, whether AC or DC, is not in the 138 kV˜765.5 kV range required for transmission. This means that either a transformer is required to step up the AC output of the wind turbine to the high voltages required before rectification, or a high-voltage DC-DC converter is required to convert the low-voltage rectified output of the wind turbine to the high voltages required.
Because transformers are big, heavy, and relatively difficult to design to precise tolerances, there has been some interest in the use of high-voltage DC-DC converters to step-up the rectified output of a wind turbine to the voltages required for transmission and/or distribution. However, conventional DC-DC converters also have their shortcomings. This can be best illustrated by looking at the conventional DC-DC converter shown in FIG. 1.
FIG. 1 is a circuit diagram of a conventional DC-DC switched-capacitor converter called a Cockcroft-Walton multiplier. A voltage input VIN supplies the bottom of a capacitor ladder made up of capacitors C1, C2, C3, and C4. A series of switches labeled S1 through S9, which in this example are insulated gate bipolar junction transistors or IGBTs, control how VIN is connected to the capacitor ladder and also control how charge may flow from capacitor to capacitor through the ladder. Each switch has a control terminal or gate that is driven by a control voltage that is labeled to indicate a phase of operation: voltages having names starting with VA are active (and their corresponding switches are closed) during a first phase of operation and voltages having names starting with VB are active (and their corresponding switches are closed) during a second phase of operation. Thus, FIG. 1, in the first phase of operation, all odd-numbered switches are closed and all even-numbered switches are open, while in the second phase of operation, all even-numbered switches are closed and all odd-numbered switches are open.
The conventional switched capacitor converter design shown in FIG. 1 suffers from several disadvantages. Each additional rung of the capacitor ladder requires an additional active switch, such as an IGBT. Since each rung of the capacitor ladder outputs a higher voltage than the previous rung, the control voltages input into the switches must be correspondingly higher as well. This requires the addition of a voltage control block, VCONTROL, which must provide a higher voltage for each subsequent rung up the capacitor ladder. For example, switch S5 has one terminal connected to the positive terminal of VIN, which means that control voltage VA1 must be higher than VIN by at least a turn-on threshold voltage (VT) in order to turn S5 on. As shown in FIG. 1, VCONTROL must provide a voltage VA1 that is at least equal to VIN+VT. Likewise, capacitor C1 will charge to a voltage VC1, which will be higher than VIN, so control voltage VB1 must be greater than or equal to VC1+VT in order to turn on. As shown in FIG. 1, VCONTROL must provide a voltage VB1 that is at least equal to VC1+VT. As additional stages are added to the top of the capacitor ladder, the control voltages provided by VCONTROL must be ever higher.
Thus, not only does the conventional switched capacitor converter design illustrated in FIG. 1 require many active switches, the design also requires a complex voltage control circuit that provides control signals with very large voltages. These two characteristics of the conventional switched capacitor converter illustrated in FIG. 1 impose a design constraint that limits the maximum output voltage that may be produced by this design.
FIG. 2 is a circuit diagram of another conventional DC-DC switched-capacitor converter that is commonly referred to as a “voltage doubler” converter. A set of switches labeled S1 through S8 are configured such that in one state, even numbered switches are closed while odd numbered switches are open, and in another state, add numbered switches are closed while even numbered switches are open. By toggling between these two states, capacitor C1 is charged to a value=Vin, and capacitor C2 is charged to a value=2×Vin. Capacitor C3 is charged to the value of C2, i.e., 2×Vin, and capacitor C4 is charged to twice the value of C2, i.e., 4×Vin.
The conventional switched capacitor converter design shown in FIG. 2 also suffers from some disadvantages, including the need to create higher control voltages such as VA1, VB2, VA3, and VB4. In addition, capacitor C2 must be rated for a voltage=2×Vin and capacitor C4 must be rated for a voltage=4×Vin. As additional voltage doubling stages are added the output capacitor for that stage must be rated for twice the voltage as the output capacitor for the previous stage. Another characteristic of the conventional DC-DC converters shown in FIGS. 1 and 2 is that Vin and Vout share the same ground node.
Accordingly, in light of these disadvantages associated with conventional converter designs, there exists a need for a high power density switched-capacitor converter.